A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe formed by a gaseous explosion. When magma rises up through a crack in the Earth's crust and makes contact with a shallow body of ground water, rapid expansion of heated water vapor and volcanic gases can cause a series of explosions. A relatively shallow crater is left (known as a maar) and a rock filled fracture (the actual diatreme) in the Earth's crust. Diatremes breach the Earth's surface and produce a steep inverted cone shape.

The term diatreme has been applied more generally to any concave body of broken rock formed by explosive or hydrostatic forces, whether or not it is related to volcanism.

Diatreme formation is sometimes associated with kimberlite magma, which originates in the upper mantle. When a diatreme is formed due to a kimberlite intrusion, there is a possibility that diamonds may be brought up, because diamonds are formed in the upper mantle at depths of 150-200 kilometers. Kimberlite magmas can sometimes include chunks of diamond as xenoliths, making them economically significant.